fix: make link_preview_timeout configurable in AdaptiveConfig

[Br1an67]

Summary

Fixes #1659

The timeout for LinkPreviewConfig in AdaptiveCrawler._crawl_with_preview was hardcoded to 5 seconds. This adds a link_preview_timeout field to AdaptiveConfig (defaults to 5.0 for backward compatibility) and wires it through.

List of files changed and why

crawl4ai/adaptive_crawler.py — Added link_preview_timeout: float = 5.0 to AdaptiveConfig and replaced the hardcoded timeout=5 with self.config.link_preview_timeout.

How Has This Been Tested?

Verified that the default value (5.0) preserves existing behavior, and that setting a custom value (e.g. AdaptiveConfig(link_preview_timeout=15)) correctly propagates to LinkPreviewConfig.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added/updated unit tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes